The meaning of DAY is the time of light between one night and the next. How to use day in a sentence. A day is a 24-hour period, or the length of time it takes the Earth to rotate fully on its axis. There are 365 days in a year, and 31 days in the month of January. On average, this is 24 hours (86,400 seconds). As a day passes at a given location it experiences morning, afternoon, evening, and night. This daily cycle drives circadian rhythms in many organisms, which are vital to many life processes. 1. Of or relating to the day. 2. Working during the day: the day nurse. 3. Occurring before nightfall: a day hike. The time taken for the Earth to make a full rotation about its axis with respect to the fixed stars; a sidereal day or stellar day. (informal or meteorology) A 24-hour period beginning at 6am or sunrise. Find out today's date, current time, day of the week, week number, and more. Your complete date and time resource. Details about today's date with count of days, weeks, and months, Sun and Moon cycles, Zodiac signs and holidays. DAY meaning: 1. a period of 24 hours, especially from twelve o'clock one night to twelve o'clock the next night…. Learn more. Constitution Day marks September 17, 1787, when 39 delegates signed the U.S. Constitution at the Philadelphia Convention — the document that established the three branches of government, the Bill of Rights, and the framework that has governed the United States for more than 235 years. DAY definition: 1. a period of 24 hours, especially from twelve o'clock one night to twelve o'clock the next night…. Learn more.